The invention relates to a diffractive security element with a half-tone image as set forth in the classifying portion of claim 1.
Such security elements are used for the authentication of documents, banknotes, passes and identity cards, valuable articles of all kinds and so forth as, although they are easy to verify, they are difficult to imitate. The security element is generally fixed by adhesive on the article to be authenticated.
It is known from EP-A 0 105 099 for a security pattern of a graphic configuration to be composed mosaic-like from diffractive image elements. The security pattern changes its appearance when the person viewing it tilts the security pattern and/or rotates the security pattern in its plane.
EP-A 0 330 738 describes security patterns which have diffractive surface portions which are smaller than 0.3 mm arranged individually or in a row in the structure of the security pattern. In particular the surface portions form text characters of a height of less than 0.3 mm. The shape of the surface portions or letters can be recognised only by means of a good magnifying glass.
It is also known from EP-A 0 375 833 for a plurality of diffractive security patterns which are composed of pixels to be disposed in a security element, wherein each of the security patterns is visible by the naked eye in a predetermined orientation at the normal reading distance. Each security pattern is divided into pixels of the raster field which is predetermined by the security element. The raster field of the security element is subdivided into diffractive surface proportions, corresponding to the number of security patterns. In each raster field the pixels of the security patterns, which are associated with the raster field, occupy their predetermined surface proportion.
German laid-open application No 1 957 475 and CH 653 782 discloses a further family of microscopically fine relief structures which have an optical-diffraction effect, using the name kinoform. The relief structure of the kinoform deflects light into a predetermined solid angle. It is only when the kinoform is illuminated with substantially coherent light that the information stored in the kinoform can be rendered visible on a display screen. The kinoform scatters white light or daylight into the solid angle which is predetermined by the kinoform, but outside that angle the kinoform surface appears dark grey.
The diffractive security pattern is enclosed in a layer composite of plastic materials, which is designed to be applied to an article. U.S. Pat. No. 4,856,857 describes various configurations of the layer composite and the appropriate materials are listed therein.
On the other hand it is known from U.S. Pat. No. 6,198,545 to form half-tone images, produced by a printing procedure, comprising pixels, with image elements or characters, wherein the black component in the otherwise white pixel background is so selected that the viewing person sees the half-tone image at the viewing distance of 30 cm to 1 m and can recognise the image elements or characters only when viewing more meticulously, at a very close distance or with a magnifying glass. That image synthesis technology is known by the term ‘artistic screening’. Good copies of half-tone images without artistic screening are easy to produce as a result of the continuously improved resolution in copying technology.